1. Field of the Invention
The present invention relates to a magneto-optic recording medium used for recording and reproducing information with a laser beam based on the magneto-optic effect. More particularly, the present invention relates to a magneto-optic recording medium utilizing the DWDD (Domain Wall Displacement Detection) technique.
2. Description of the Related Art
Hitherto, as one method for realizing high-density recording in a rewritable manner, attention has been focused upon a magneto-optic recording medium on which information is recorded by writing magnetic domains in a magnetic thin film with thermal energy of a semiconductor laser, and from which the recorded information is read based on the magneto-optic effect. In recent years, because of the necessity of handling data containing in formation in various forms, such as voices, still images and motion images, and because of the ever increasing size of such data to be handled, the demand has grown for increasing the recording density of a magneto-optic recording medium and providing a recording medium with a larger capacity.
Generally, the recording density of a magneto-optic recording medium, such as a magneto-optic (optical) disk, greatly depends upon the laser wavelength of a reproduction optical system and the numerical aperture NA of an objective lens. In other words, once the laser wavelength xcex of a reproduction optical system and the numerical aperture NA of an objective lens are decided, the diameter of the beam width is also decided. Hence, the spatial frequency of recording pits, which can be reproduced as a signal, has an upper limit of about 2NA/xcex.
To realize a higher density in conventional optical discs, therefore, it is required to shorten the laser wavelength of a reproduction optical system or to increase the numerical aperture of an objective lens. However, shortening the laser wavelength is difficult because of problems such as efficiency and heat generation of a laser device. Also, if the numerical aperture of an objective lens is increased, the distance between the lens and the disk becomes so small as to cause a mechanical problem in that the disk may strike the lens.
Meanwhile, the so-called magnetic ultra-resolution technique has been developed with the intent to increase the recording density by improving the structure of a recording medium and the reproducing method. For example, Japanese Patent Laid-Open No. 7-334877 proposes an ultra-resolution method. According to this proposed method, a memory layer for holding recording information, a reproduction layer for masking a part of a reproduction light spot, and a cutoff layer for controlling an exchange coupling force between the memory layer and the reproduction layer are formed in a multilayered structure. The recording information is transferred to the reproduction layer essentially in the part of the reproduction light spot by utilizing a temperature distribution generated in the medium upon irradiation of the spot, whereby the recording information is reproduced from a minute magnetic domain.
However, the above conventional ultra-resolution method has a problem in that the light in a masked portion is wasted and the amplitude of a reproduced signal is reduced, because the resolution power is increased when masking a part of the reproduction light spot based on a temperature distribution; namely, by essentially restricting an aperture for reading a pit to a smaller area. Stated otherwise, since the light in the masked portion does not contribute to generating the reproduced signal, the amount of effectively available light is reduced as the aperture is narrowed to improve resolution. This results in a lowered signal level.
Also, Japanese Patent Laid-Open No. 6-290496 discloses a method in which a displacement layer having a small domain wall coercive force is provided on the incident side of reproduction light relative to a memory layer, and a domain wall in the displacement layer is displaced toward the higher temperature side based on a temperature gradient generated in a reproduction light spot. This reproduces information from a magnetic domain while enlarging it within the spot. According to this disclosed method, since a signal is reproduced while enlarging a magnetic domain, the reproduction light can be effectively used even when a recording mark has a smaller size. Hence, the resolution power can be increased without deteriorating the amplitude of the reproduced signal.
With recent improvements in information processing technology, not only is large capacity required, but also high-speed recording/reproducing techniques are demanded in the field of large-capacity recording media. The method disclosed in the above-cited Japanese Patent Laid-Open No. 6-290496 presents a novel approach by increasing the medium capacity through a process of displacing a domain wall based on a temperature gradient generated in a reproduction light spot and reproducing information from a magnetic domain while enlarging it within the spot. However, the disclosed method does not consider its impact on transfer rate. The method causes a problem in that, depending on the structure of a recording medium, the quality of a reproduced signal is greatly deteriorated when the medium is moved at high linear speeds.
Further, when trying to produce a temperature gradient for displacing a domain wall by heating a recording medium with a reproduction laser beam itself, the peak of a temperature distribution is formed within the reproduction light spot. Therefore, such an attempt raises the problem that domain walls are displaced toward the reproduction light spot respectively from the upstream (forward) and downstream (backward) sides in the moving direction of the recording medium relative to the reproduction light spot and are read as leaked signals with the spot. This makes it difficult to obtain a good reproduced signal. To overcome that problem, it has been proposed to provide a means for producing a desired temperature distribution separately from the reproduction laser beam. This solution, however, raises another problem in that it makes a reproducing apparatus complicated.
Japanese Patent Laid-Open No. 11-86372 discloses a method of restraining displacement of a domain wall from the downstream side by applying a reproduction magnetic field. However, the disclosed method requires a means for separately adding the reproduction magnetic field and also makes the reproducing apparatus complicated.
With the view of overcoming the problems mentioned above, it is an object of the present invention to provide a magneto-optic recording medium and a reproducing method and apparatus, by which a signal with a recording density in excess of the resolution of an optical system can be reproduced at high speed without making the reproducing apparatus and the medium structure complicated.
The above object is achieved with a magneto-optic recording medium of the present invention constructed as follows.
In a magneto-optic recording medium in which information is reproduced while a recording magnetic domain is enlarged by displacing a domain wall based on a temperature distribution formed by a beam of light and having a maximum temperature Tr, the recording medium comprises a reproducing layer in which a domain wall is displaced; a recording layer for holding a recording magnetic domain corresponding to information; and a cutoff layer disposed between the reproducing layer and the recording layer and having a Curie temperature lower than those of the reproducing layer and the recording layer, wherein the recording medium satisfies the following condition;
(Trxe2x88x92RT)/(Tc2xe2x88x92RT)xe2x89xa71.8 
where
Tc2: Curie temperature of the cutoff layer, and
RT: room temperature.
Also, the above object is achieved with a reproducing method and apparatus for reproducing recorded information from the magneto-optic recording medium set forth above. Specifically, the reproducing method includes reproducing recorded information form a magneto-optical recording medium comprising recording information on the recording layer and thereafter reproducing the information while displacing the domain wall by the beam of light to enlarge the recording magnetic domain and the apparatus includes an optical disk comprising a substrate having a tracking guide groove and the magneto-optical recording medium of the invention formed thereon.